Overlapping coverage issues with 802.11n BSSs April 2006 Overlapping coverage issues with 802.11n BSSs M. Benveniste M. Benveniste (Avaya Labs)
Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Abstract Use of an extension channel should not lower aggregate throughput of a WLAN system (relative to having no extension channel) Channel sharing by OBSS, enabled by CSMA/CA and virtual carrier sensing, addresses the limited availability of channels Avoiding collisions in OBSS through RF resource management (i.e. restricting channel sharing by BSSs with coverage overlap) could decrease aggregate throughput Virtual carrier is important for both control and extension channels M. Benveniste (Avaya Labs) John Doe, His Company
Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Overlapping Coverage With 40 MHz tx, BSSs with coverage overlap may share one or both of their channels 4 overlap situations: BSSs use the same control and extension channels BSSs use same extension channel but different control channel The control channel of one BSS is the extension channel of the other BSSs use the same control channel but different extension channels When one channel is shared, we have POBSS = partially overlapping BSS M. Benveniste (Avaya Labs) John Doe, His Company
Virtual Carrier Sensing – RTS/CTS Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Virtual Carrier Sensing – RTS/CTS RTS/CTS helps avoid collisions between 20 MHz tx in OBSSs (using the same control channel) RTS/CTS can prevent collisions with 40 MHz tx only in POBSS cases 1 and 4 POBSS cases 2 and 3 would be more common M. Benveniste (Avaya Labs) John Doe, His Company
Channel sharing by BSSs with overlapping coverage Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Channel sharing by BSSs with overlapping coverage The available channels are not enough to provide all BSSs with overlapping coverage a separate channel BSSs must share channels; CSMA/CA makes that easy Since the extension channel is used less heavily than the control channel, it is better for BSSs with coverage overlap to share the same extension channel use one’s control channel as the other’s extension channel M. Benveniste (Avaya Labs) John Doe, His Company
Sharing extension channel – POBSS Case 2 – results in collisions Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Sharing extension channel – POBSS Case 2 – results in collisions BSSs A and B have coverage overlap; they use different control channels, but the same extension channel AP A hears no transmission when station 1 is transmitting on the extension channel A 40 MHz transmission when CCA (on the control channel) indicates inactivity on extension channel would cause collision with station 1’s tx 1 A B M. Benveniste (Avaya Labs) John Doe, His Company
Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Sharing control/extension channel – POBSS Case 3 – results in collisions BSSs A and B have coverage overlap; A uses as control channel B’s extension channel If station 1 is transmitting on the extension channel, AP A hears no transmission A 40 MHz transmission when CCA (on the control channel) indicates inactivity on extension channel would cause collision with station 1’s tx 1 A B M. Benveniste (Avaya Labs) John Doe, His Company
Virtual Carrier Sense for POBSS Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Virtual Carrier Sense for POBSS To avoid collisions in POBSSs, the extension channel should be monitored independently RTS/CTS on both the control extension channel should be decoded within interference range and NAVs should be maintained M. Benveniste (Avaya Labs) John Doe, His Company
Month 2002 doc.: IEEE 802.11-02/xxxr0 April 2006 Conclusions Use of an extension channel should not lower aggregate throughput of a WLAN system (relative to no extension channel) Maintaining a NAV on both the extension and the control channel is desirable, as it enables channel sharing by POBSS with collision avoidance M. Benveniste (Avaya Labs) John Doe, His Company